1. Field of the Invention
This invention relates to a method of recording information at high density and a method of reading the recorded information.
2. Description of the Prior Art
Known conventional methods of high density recording of information are the magnetic recording causing a difference in magnetization with a magnetic head, the magneto-optical recording causing a difference in magnetization with an optical head and the method causing a chemical change or physical shape change through photochemical or opto-thermal action.
The recording density obtained through these techniques is univocally limited by the size of the recording head. Namely, the gap area of the smallest magnetic head produced under the existing technology is 3.5 .mu.m.sup.2 (the track pitch is 10 .mu.m and the gap width is 0.35 .mu.m). On the other hand, for the optical head, the spot diameter d of a light beam is expressed as: ##EQU1## where .lambda. is the wave length of the light beam and NA is the numerical aperture of a lens in the head. Therefore, if the NA of a frequently-used lens is 0.5 and the wave length .lambda. is 0.8 .mu.m, then the spot diameter becomes d.apprxeq.1.6 .mu.m (the spot area is 2 .mu.m.sup.2). With these heads, the two-dimensional recording density becomes 10.sup.8 bits/cm.sup.2 :
In order to achieve a still higher density storage of information, a memory device which utilizes ion beams for writing and reading is known (F. W. Martin, U.S. Pat. No. 4,088,895). This device stores information on an extremely small scale by change of the physical condition of a substrate when the substrate is irradiated by ion beam. When the beam irradiates, a memory site is formed which corresponds to a binary "1" bit, while the absence of such a site corresponds to a binary "0". Information is stored in an array of such sites, for example in a rectangular array in which each site is addressed by the horizontal and vertical distances from a fiducial starting point on the substrate surface. The presence or absence of irradiation on a site is determined by ion beam. The ions interact differently with the site where the ion beam struck. The current of the ion beam utilized is 10 to 1,000 picoamperes while its beam diameter is smaller than 1 .mu.m. When utilizing a 300 pA, 2,000 .ANG. radius beam, it takes 10 .mu.sec to write one memory site. In reading, the same type of ion or another type of ion from another irradiation mechanism is irradiated, and ultraviolet light or X-ray photon generated thereby is detected.
Another known method utilizes a semiconducting diode as a substrate to read the current increase of the semiconducting diode resulting from ion implementation to its depletion region (C. G. Kirkpatrick, U.S. Pat. No. 4,130,891).
In the above-mentioned prior art, since the same phenomenon as that in writing occurs in reading, the clarity of written information, or the signal-to-noise ratio, will reduce by repetitive reading operations. Further, in both of the latter two methods, since the written information cannot be erased, they can be utilized only as a read only memory.